It is now common for businesses and homeowners to have a security system for reporting of alarm conditions to a security station. One of the primary functions of such a security system is to provide an alarm report to the security station when the owner is absent and one or more of the detectors reports an alarm condition. The type of detectors vary from relatively simple hard wired detectors, such as door or window contacts, to more sophisticated battery operated sensors such as motion and glass break detectors.
The security alarm system located in the premise being protected includes a series of different types of sensors all of which report to an alarm control panel. The control panel is typically installed in a safe location and is directly connected to the power supply. The alarm control panel is connected either by hardwires to the individual sensors or communicates or receives signals from individual detectors as part of a wireless communication. The communication between the alarm panel and the sensors can be a one way communication where the detectors merely transmit to the alarm panel or can be two way communication.
It is known that certain natural events can cause some sensors, in particular motion sensors and glass breakage sensors to falsely trigger. Lightning produces a broadband electromagnetic signal which produces a signal in the detector or causes a change in the monitored space leading to a false activation of the detector. Thunder associated with lightning can also cause a large acoustic signal which can lead to false alarms. Lightning and thunder are two examples of naturally occurring events which produce high energy which can lead to false alarms in security detectors.
The occurrence of false alarms is a major concern to the security industry. As can be appreciated, naturally occurring events such as lightning typically do not affect a single alarm system but affect a number of alarm systems in a certain proximity to the natural occurring event. The alarm panels report to the security system the detection of an alarm condition which is then processed by the security station. Processing of each false alarm incurs a certain expense but more importantly, it also reduces the confidence that a reported alarm condition is in fact an alarm condition which requires prompt investigation.
To overcome the above problems, some motion detectors have been provided with additional circuitry for detecting a naturally occurring event such as lightning. U.S. Pat. No. 5,920,259 discloses a motion detector unit having additional circuitry for detecting the occurrence of lightning. Basically, the detector circuitry acts as a receiver and produces a signal of a magnitude to the sensor when exposed to the electromagnetic signal caused by lightning. To overcome this problem, each detector is provided with redundant circuitry which is also affected by this electromagnetic interference. By subtracting the signal from the motion sensor circuitry from the signal of the redundant circuit, the effect of the electromagnetic signal is eliminated or reduced.
Unfortunately, this arrangement requires each motion detector to include additional redundant circuitry and additional processing of the signals from the two different circuits. This significantly adds to the cost of each sensor, thereby increasing the cost of the overall system.
U.S. Pat. No. 5,977,762 discloses a lightning detector suitable for protection of electronic equipment in a home. The detector receives an electromagnetic signal, through an antennae and applies signal conditioning to produce a series of pulses corresponding to peaks in the signal above a specified threshold. The pulse data is compared with a predetermined criteria that distinguishes lightning produced interference from other electrical interference.
It has been recognized that certain natural occurring events produce electromagnetic interference or radio frequency interference which negatively affect the performance of motion detectors and security systems.
There remains a need to provide a cost effective solution for reducing or modifying the processing of signals from an alarm system when such interference is detected.